The Boulder Barricade at Cap a la Baleine, North Shore of Gaspe Peninsula (Quebec): Nature of Boulders, Origin and Significance

Abstract

A small (1.5 km long) boulder barricade occurs at Cap a la Baleine on the north shore of Gaspe Peninsula, about 25 km NE of Matane, The boulder barricade is a low ridge at the low tide level formed by a pavement of small to medium size subrounded to rounded clasts of various lithologies. Boulders are closely packed together and compacted into the underlying clay substrate. There are two main groups of boulders. About 40% are Precambrian erratics from the Laurentidian Shield on the north shore of the St. Lawrence estuary, whereas the remaining are from Appalachian lithologies occurring on the south shore of the St. Lawrence, of which 54% are sandstone (graywacke). A few particular indicators are anorthosite, dolostone, quartzite and volcanic erratics. Boulders remaining on the tidal flat surface are a lag from the erosion of a clay diamict, an event which occurred during the Upper Holocene. Originally deposited in the Goldthwait Sea circa 13-13.5 ka, the boulders are most likely iceberg drifted erratics from two sources: the Laurentides Ice Sheet still occupying the St. Lawrence valley, and the Appalachian ice, the front of which corresponding to the present shoreline at the time. Today, the Cap a la Baleine boulder barricade forms a natural wave breaker slowing erosion of the tidal flat.